High power fibre lasers require multi-mode pump combiners to couple pump radiation from multiple high power fibre-coupled diodes into the cladding of rare-earth-doped gain fibres, either directly, or via an intermediate fibre. These fibres are often coated with a polymer coating which acts as a cladding for the pump radiation to be guided against. However, this polymer coating is a source of unreliability as it can overheat if subjected to pump radiation that exceeds the numerical aperture of the fibre.
It would be advantageous if there were a high power combiner in which light with a high numerical aperture could be removed from light with the desired lower numerical aperture such that subsequent absorption by the high numerical aperture polymer coating of the intermediary or gain fibre is minimized. It would be advantageous if this could be achieved without the use of polymers, liquids, gels, or other materials that can thermally degrade or that have a refractive index that is more sensitive to temperature than silica. Such a solution would enable additional power-scaling of fibre lasers by permitting higher pump powers to be used and allowing for gain fibres with smaller cladding areas cladding sizes. This is advantageous because it would enable the non-linear power threshold of the fibre laser to be increased, permitting either shorter lengths of gain fibres at the same power levels, or higher output power levels if the same length of gain fibre is used.
U.S. Patent Application Publication No. US2015-0062693 describes a high-power liquid-cooled pump and signal combiner that uses a fluid within a cooling chamber. The fluid has a refractive index selected to control the interaction and propagation of the light in the fluid. This method of removing unwanted light is complex and expensive to implement. Any method of introducing a liquid into a compact and sensitive component will require careful engineering. Further, the control of the refractive index of the liquid will be critical in determining the performance of the signal combiner. The refractive index (and so the composition) of the liquid will need to vary depending on the specific design of the combiner and the characteristics of the inputs to the combiner. There are also concerns about the long term reliability of components having bare fibre with exposed glass surfaces in contract with the fluid.
U.S. Patent Application Publication No. US2014-0241663 describes an ultra-high power fiber laser system including a multimode combiner which is configured with a plurality of low mode fibers bundled together and tapering toward its downstream end. The system further includes a clad mode absorber extending along the tapered downstream end of the combiner and extending over a portion of the combiner's output fiber. The absorber is configured with sequentially located zones which are provided with respective refractive indices. In a forward propagating direction of light signal, the upstream zone includes polymeric material with the refractive index higher than that of the cladding of the combiner end fiber. This zone is configured to remove the back reflected core guided light bled into the cladding of the combiner through a splice between combiner end and output fibers. The intermediate zone includes polymeric material configured with a refractive index lower than that of the cladding of the combiner output fiber so it can prevent clad guided signal light from decoupling the cladding under the material. The downstream zone is configured with polymeric material having a refractive index lower than that of the cladding of the combiner output fiber. The polymeric material of the downstream zone is impregnated with a plurality of light diffusers scattering high numerical aperture rays of the clad-guided signal light. The multimode combiner, however, is based on polymeric materials which have associated reliability concerns owing to poor thermal properties.
An aim of the present invention is to provide an apparatus for combining optical radiation which reduces the above aforementioned problems.